1. Technical Field
The present invention generally relates to fluid flow metering and control devices, and more particularly relates to software related correction methods for such flow devices.
2. Related Art
In process control industries, it is common to use small diameter tubes to carry process fluids at low flow rates when small amounts of fluids are required for manufacturing processes. The tubes are almost always of a circular cross-section. Instruments used to measure a flow rate in the tubes must interface with a fluid flowing in the tube while minimizing disturbance to the fluid flow. To minimize disturbance to the fluid flow, the instrument typically includes a circular cross-section to match the cross-section of the tubes. The flow rate for a flow meter measuring a change in pressure across an orifice is defined by the following Equation 1:
                    Q        =                  C          ·                      A            o                    ·                                    (                              1                                  1                  -                                                            (                                                                        A                          o                                                                          A                          P                                                                    )                                        2                                                              )                                      1              2                                ·                                    (                              2                ·                                                      (                                                                  P                        hi                                            -                                              P                        lo                                                              )                                    ρ                                            )                                      1              2                                                          Equation        ⁢                                  ⁢        1            
Where:                Q=volumetric flow rate        C=orifice discharge coefficient        Ao=cross-sectional area of the orifice        Ap=cross-sectional area of the pipe        Phi=upstream pressure        Plo=downstream pressure        ρ=density of the fluidThe differential pressure measurement (Phi−Plo) could be made using two individual pressure measurements and combining them to get the pressure difference or pressure drop or using a single device as represented in FIG. 15.        
When orifices and differential pressure measurements are used to calculate flow through large pipes it is common for them to be discrete devices that are bolted or otherwise attached to the pipe. There are also devices available for measuring the flow in small tubes that have the orifice and pressure sensors integrated into the same housing. In almost all cases, the measuring device orifices are of a fixed size for measuring flow over a fixed flow range. The flow characteristic or “discharge coefficient” of the orifice is measured, or determined by design, by the manufacturer. For discrete systems, the end user may calculate the flow based upon the parameters in Equation 1, including a manufacturer provided discharge coefficient. In integrated systems, the discharge coefficient can simply be accounted for as part of a total device calibration performed by the manufacturer.
Differential pressure orifice flow metering is most accurate when the flow rate is near the upper end of the flow range that the meter is designed for; that is, where the pressure change is relatively large for a given change in flow rate. As the flow rate decreases, the accuracy of the device decreases because there is a relatively small pressure change for a given change in flow rate. This phenomena can also be described as a decrease in the differential pressure to flow rate ratio, which ratio is shown in the graph of FIG. 16. Since the pressure differential must be accurately known to calculate the flow rate, any error in the differential pressure measurement causes an error in the flow calculation. As the slope of the curve gets steeper at low flow rates (see FIG. 16), any pressure measurement error causes a larger flow calculation error.
In order to make more accurate flow measurements over a larger range of flow rates using an orifice and differential pressure measurement, it may be advantageous to use a variable-sized orifice. A variable-sized orifice can be used to improve the flow measurement accuracy over the range of orifice openings by providing a relatively high pressure differential for each flow rate. However, even though computational fluid dynamics (CFD) software can be used to optimize the design of a variable-sized orifice, there is still a small change in the discharge coefficient as the size of the orifice is varied. This change is due to the range of flows that the device is designed to measure, and the physical factors that contribute to the discharge coefficient of an orifice.
Some variable-sized orifice devices are designed to cover flow ranges that begin in the laminar flow region and end in the turbulent flow region, which make it likely that the discharge coefficient will vary in the different flow ranges. Also, it is known that the discharge coefficient of an orifice is comprised of a combination of physical effects relating to the fluid and the shape of the orifice. When the orifice is set for a very small opening, the surface area of the walls of the flow path are large relative to the cross-sectional area of the flow path. This is because a “slit” type opening results. In a slit type opening, the viscous force of the liquid against the walls in the orifice region of the flow path becomes much more significant than when a larger opening is present. A larger ratio of the wall surface area to the flow path cross-sectional area has the effect of lowering the discharge coefficient of the orifice.
Although a variable orifice flow meter may have the advantage of extending the range of a flow meter by as much as a factor of 10, it may have the inherent drawback of decreased accuracy due to slight changes in the discharge coefficient at different openings, and for different flow rates at any given opening size.
In addition to the above noted disadvantages related to discharge coefficients, known variable orifice devices are ineffective for several other reasons. First, known variable orifice devices typically use circular or curved members that are moved with respect to the fluid flow to change the size of the orifice. Because of the curved nature of these members, the shape of the orifice changes as the size of the orifice changes, which results in significant errors when calculating the fluid flow over a range of orifice sizes. Second, the changed shape of the orifice leads to non-ideal orifice shapes for at least a portion of the flow range. This leads to inconsistent flow characteristics for any given opening as flow rate changes, again leading to errors in the calculation of fluid flow.
A flow device that addresses these and other shortcomings of known flow control and metering devices would be an important advance in the art.